In processes which rely on delivery of large amounts of heat energy into a furnace by combustion of a fuel, it is particularly important to achieve as high an energy-efficiency as possible. Thus it is a common practice to recover excess heat in the flue-gas, for example by using it to heat combustion air. Another way to improve efficiency is by oxy-combustion, which, by replacing air ordinarily used in combustion with a stream that is largely oxygen, avoids heating the nitrogen component of air. While heat lost to the flue gas is reduced in oxy-combustion (because the flue-gas volume is less), the amount of heat lost is still substantial, and it would be advantageous to recover that heat.
Heat recovery from oxycombustion in glass-making was discussed in detail in a study titled “Development of an advanced glass melting system: The Thermally Efficient Alternative Melter, TEAM. Phase 1, Final report.—Progress rept.” that was funded by the U.S. Department of Energy (DOE) and completed by Air Products under contract DE-AC02-89CE40917F (report number DOE/CE/40917-T2 published in February 1992 and available from the National Technical Information Service of the U.S. government). In this report, the primary methods discussed were: 1) batch and cullet (i.e. glass-making raw materials) pre-heating, 2) natural gas reforming with steam or CO2, and 3) gas turbine cycles (where heat is converted to gas compression for an air separation unit). Of these, the batch and cullet pre-heating options were found to offer the greatest improvement in efficiency. However, feedback from glassmakers indicated that batch and cullet pre-heating approaches had been tried and were found extremely difficult due to clogging, mechanical complexity, and continual maintenance problems. Natural gas reforming with steam was found to offer good efficiency improvement and was recommended for further development.
In the automotive field, heat exchangers are known that allow transfer of heat energy to multiple independent fluid flows. Such a heat exchanger is described by US2006/0266501. These are plate-type heat exchangers, which are not suitable for oxygen service for multiple reasons. For example, this type of heat exchanger does not provide a sufficiently smooth flow path and is too difficult to manufacture with adequate cleanliness for oxygen service, and especially for hot oxygen service. Cleanliness for hot oxygen service is of course greatly important for safety reasons since the local highly oxidizing environment can pose an unacceptably high risk of uncontrolled combustion.
U.S. Pat. No. 5,006,141 discloses cullet pre-heating schemes including ones where both cullet pre-heating and fuel pre-heating are combined. These schemes are subject to the difficulties of cullet pre-heating discussed above.
U.S. Pat. No. 5,057,133 discloses a natural gas reforming scheme whereby flue gas used to provide heat to the reformer is combined with a recycle stream from downstream of the reformer, in order to provide temperature control. Using a fluidized bed of hot sand to capture condensibles and recycling the sand to the glass-making furnace is also described.
U.S. Pat. No. 5,714,132 discloses natural gas reforming using the flue gas itself as a source of steam and/or CO2. While this concept appears attractive in principle, in practice, sulfur and other catalyst poisons in the flue gas are difficult to economically remove.
Although natural gas reforming was considered promising for heat recovery, it has not been implemented. Whereas steam reforming is a well-known process, this application requires additional development, notably reformer tubes compatible with the condensation of sulfates and borates expected in the glass flue-gas stream, and burner technology suited for the lower-energy-density fuel. These hurdles have proved too daunting to allow the practical application of the concept.
Also mentioned in the above DOE report are the concepts of heating both the oxygen and natural gas streams. The temperature limit for O2 pre-heating is given as 465° F. (240° C.), imposed by material compatibility with hot oxygen, while that for natural gas pre-heating is given as 750° F. (399° C.), imposed by the thermal cracking (carbon formation) of natural gas at higher temperatures.
An alternative to the schemes discussed above is the heating of oxygen to temperatures above 240° C. However, heating the oxygen stream is extremely challenging, because the high reactivity of oxygen, especially at high temperature, places extreme constraints on the design and construction of the heat recovery system. For example, while it is a common practice to use a regenerator, through which flue gas and air flow in alternating cycles in order to preheat the air, it is generally considered impossible to use this technique with oxygen because of the fear that oxygen would react with contaminants inevitably present in the flue gas and deposited in the regenerator.
Another known solution is the use of ceramic heat exchangers. These systems are usually intended to operate at temperatures of about 1000° C., where heat transfer is radiative. However, ceramic materials are known to be fragile and ceramic heat exchangers are prone to leakage. Whereas minor leakage of air into the flue gas stream is acceptable, this is not the case for oxygen or fuel gas streams due to safety issues. Thus, heat exchangers of this type are not acceptable for heating oxygen or fuel gas streams.
Thus, there is a need to provide a method and system for the recovery of heat from products of combustion that is robust and not susceptible to leaks causing safety issues.
US 2009/0298002 discloses the use of a shell and tube heat exchanger where oxygen flows through the double-walled tubes while hot combustion gases flow through the shell. Contact between the hot combustion gases, which may include unburnt fuel, and the oxygen is inhibited not only by the presence of the walls of the inner tubes but also by the presence of the walls of the outer tubes. The annular spaces between the ducts and tubes contain a static inert gas so that heat exchange first proceeds between the hot combustion gases and the inert gas and then between the inert gas and the oxygen. US 2009/0298002 does not address how the novel heat exchanger may be used in a furnace having multiple burners. Moreover, it suffers from the disadvantage of exhibiting a relatively lower heat exchange coefficient because each of the two separate phases of heat exchange take place between a flowing fluid and a static fluid.
Thus, there is a need to provide a method and system for the recovery of heat from products of combustion that includes multiple burners and which has a sufficiently high heat exchange coefficient.
U.S. Pat. No. 5,807,418 discloses heat recovery by “co-current indirect heat exchange” of an oxidant (at least 50% O2) by the flue-gas, followed by using the partially-cooled flue-gas to pre-heat batch and/or cullet. As used by U.S. Pat. No. 5,807,418, “co-current indirect heat exchange” refers simply to a heat exchanger in which the oxidant and heat exchanger are separated by a wall, with both the oxidant and the flue gas flowing in the same direction. While a sketch is provided, details such as materials of construction of the heat exchanger are not, but for the comment that the heat exchanger is “constructed using materials and in a way that renders it compatible with and safe for handling oxygen-rich oxidants and high temperatures”. Considering the practical difficulty of constructing such a heat exchanger, this instruction is not sufficient to allow practical implementation by the skilled artisan.
Thus, there is a need to provide a method and system for the recovery of heat from products of combustion that allows practical implementation by one of ordinary skill in the art.
US 2009/0084140 uses a scheme similar to U.S. Pat. No. 5,807,418, but with batch/cullet pre-heat in parallel with oxidant pre-heat, and with additional disclosure related to the batch/cullet heat exchanger. Again, no details on the construction of the oxidant heat exchanger are disclosed. As best shown in FIG. 1, hot combustion gas FG is used to preheat oxygen OM at a heat exchanger HX. The hot oxygen is split into three streams OA, OB, OC each one of which is combusted with a fuel stream F at an associated burner B to produce the hot combustion gas FG. This approach suffers from the disadvantage that the flow rates of the individual oxygen streams can only be separately controlled downstream of the oxygen heat exchanger. This means that the flow control devices are subjected to hot oxygen attack, leading to premature and potential catastrophic failure. This approach also suffers from the disadvantage that unburnt fuel in the hot combustion gases may come into contact with oxygen, either from a leak or at a regenerator, thereby posing an unacceptably high risk of catastrophic uncontrolled combustion.
Thus, there is a need to provide a method and system for the recovery of heat from products of combustion that allows separate control of flow rates of hot oxygen to multiple burners from a single heat exchanger that does not exhibit an unacceptably high risk of premature and potential catastrophic failure.
In order to provide a practical method for heating oxygen with flue gas, the concept of using a heat transfer fluid was discussed by Illy et al. (International Glass Journal, 96, pp 65-72, 1998), for the case of a glass-melting furnace. For the sake of clarity, it should be noted that Illy et al. refer to a heat exchanger in which the flue gas and oxidant are separated only by a wall as “direct”, whereas Chamberland et al. refer to it as “indirect”. Illy et al. discloses a scheme using three heat exchangers: one to transfer heat from flue gas to a heat transfer fluid, a second to transfer from the heated fluid to oxygen, and a third to transfer from the heated fluid air to natural gas fuel. According to their description, the heat transfer fluid might be helium using a closed loop recycling system, but could be any gas such as steam or air, with air being the least expensive option. Illy et al. did not consider how hot oxygen flow would be controlled downstream of the heat exchangers.
One solution commercially implemented utilized, on a per burner basis, includes one heat exchanger for preheating oxygen and one heat exchanger for preheating natural gas. The oxygen and natural gas are preheated in the heat exchanger against a flow of hot air that is itself heated against hot combustion gases in a recuperator. While this approach has produced desirable heat recoveries, the high number of heat exchangers can drive up capital expense to an undesirably high level when the price of metals suitable for oxygen service is itself high. Additionally, in small to medium sized furnaces, the available space may not be adequate for accommodating the large footprint taken up by the high number of heat exchangers.
Thus, there is a need to provide a method and system of heat recovery from products of combustion that does not produce an unacceptably high capital expense or present an unsatisfactorily large footprint.
US 2007/0287107 discloses one solution to the problem of control of oxygen flow when using hot oxygen. Two oxidants are delivered where first is heated to at least 300° C. and the second is maintained at 200° C. or lower. The drawback of this method is that a substantial fraction of the oxygen flow is not significantly heated and thus the recovery of heat from the flue gas is limited.
Thus, there is a need to provide a method and system of heat recovery from products of combustion that achieves a satisfactorily high degree of heat recovery.
Another solution is to use multiple heat exchangers, preferably one heat exchanger per burner, but at least one heat exchanger per 3 burners. This solution is described in US2010/0258263 and WO2008/141937. This approach results in a very high capital expense because of the need to have one heat exchanger for oxygen per 1-3 burners as well as one heat exchanger for fuel per 1-3 burners. Moreover, the large number of heat exchangers as a whole consumes a lot of space, as each heat exchanger is rather large.
Thus, there is a need to provide a method and system of heat recovery from products of combustion that does not require a very high capital expense and does not consume an undesirably high amount of space.
U.S. Pat. No. 6,250,916 discloses one solution where hot combustion gas is used to preheat air which is used, in turn, to preheat oxygen. In one embodiment, and as best illustrated in FIG. 2, each one of several burners B is associated with one heat exchanger HXO for preheating oxygen OC and one heat exchanger HXF for preheating fuel FC. The preheated oxygen OH and preheated fuel FH are combusted at the burners B to produce the hot combustion gases FG. Air A is heat exchanged with hot combustion gases FG at a recuperator R and directed in parallel to the heat exchangers HXO, HXF as multiple streams equal in number to the number of burners. Similar to US2010/0258263 and WO2008/141937, this approach also results in a very high capital expense and consumes a lot of space.
In another embodiment of U.S. Pat. No. 6,250,916, and as best illustrated in FIG. 3, air A is preheated with hot combustion gases FG at a recuperator R and directed through three heat exchangers HXO, in series, for preheating oxygen OC. The preheated oxygen OH from each heat exchanger HXO is split into two lines each one of which is directed to one of two burners B in a pair of burners B. The cooler air exiting the last heat exchanger HXO for preheating oxygen OC is then directed through three heat exchangers HXF, again in series, for preheating fuel FC. The preheated fuel FH from each heat exchanger HXF is split into two lines each one of which is directed to one of two burners B in a pair of burners B. Similar to US 2009/0084140, in order to have separate control of the flow rate of oxygen for each burner in a pair of burners, flow control devices must be located downstream of the heat exchangers, thereby subjecting them to hot oxygen attach and raising the potential for premature and potential catastrophic failure. While the oxygen flow rate for each burner in a pair of burners may be fixed, and thereby significantly decrease the ability to control heat flux within the furnace, the ratio of heat exchangers to burners is still as high as 1:2. Thus, this approach does not achieve a sufficiently desirable reduction in capital cost and space requirements.
Thus, there is a need to provide a method and system of heat recovery from products of combustion that allows a greater degree of control over the individual flow rates of burner oxygen without an unacceptably high risk of premature and potentially catastrophic failure of the flow rate control devices and without incurring an unacceptably high capital expense.